pseudo-C2 Symmetry Research Articles

Synthetic studies towards volvalerenol A: access to a fully functionalized cycloheptane framework in an asymmetric fashion through the exploitation of C2-symmetry.

The enantioselective synthesis of a fully functionalized cycloheptane core of the naturally occurring triterpenoid volvalerenol A, exhibiting pseudo-C2 symmetry, was achieved. Enantioselective enzymatic desymmetrization (EED), asymmetric methallylation, and reductive ring opening of an cyclopropane overbred intermediate were the key reactions to access the cycloheptanone core. Further synthetic manipulations, via a unique "MPV" (Meerwein-Ponndorf-Verley) type reductive ring-opening of an epoxide and other synthetic transformations, afforded two fully functionalized cycloheptane frameworks of the target molecule.

Application of amber suppression to study the role of Tyr M210 in electron transfer in R. sphaeroides photosynthetic reaction centers.

The bacterial photosynthetic reaction center (RC) is an important system to understand the structure and chemistry of the biological process of photosynthesis. In wild type reaction centers and at room temperature, the unidirectional electron transfer from the excited-primary donor P∗ to the A-side bacteriopheophytin HA takes place via a two-step mechanism, where P∗ transfers an electron to the A-side bacteriochlorphyll BA in 3 ps making P+BA-, which then transfers an electron in 1 ps to HA to make P+HA-. Even though RCs have a pseudo-C2 symmetry, electron transfer only occurs along the A-branch. Tyrosine M210 has been a key amino acid of particular interest since it is a clear deviation in symmetry between the two sides of RC, and many mutations have been made at this site. One interesting finding was that when M210 is mutated to phenylalanine, the electron transfer from P∗ and HA slows down from ∼4 ps to ∼11 ps and becomes temperature independent (Nagarajan et al. 1990, PNAS). By using amber suppression, we were able to incorporate phenylalanine variants with different electron-withdrawing/donating capabilities at position M210. To our surprise, transient absorption data shows that YM210F RCs do not exhibit temperature independent primary charge separation. In fact, YM210F RCs and all M210 phenylalanine variants RCs exhibit Arrhenius temperature dependence in contrast to WT, wherein the rate increases at low temperature. Significantly, the 1020 nm band, which is characteristic of P+BA-, was not observed at any time for any of the M210 phenylalanine variants RCs nor YM210F RCs, indicating a change to the one-step superexchange mechanism for electron transfer.

Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications

Vibrational coherence in the metal–metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C2 symmetry was investigated where two nearly degenerate transitions from the highest occupied molecular orbital (metal–metal σ* orbital) to higher energy ligand π* orbitals could be simultaneously induced. We observed oscillatory features in femtosecond degenerate transient absorption (TA) signals from complexes [(ppy)Pt(μ-tBu2pz)]2 (1) and anti-[(ppy)Pt(μ-pyt)]2 (2), which are attributed to coherent nuclear motions that modulate the HOMO (antibonding σ*) energy level, and hence, the energy for the MMLCT transition. The characteristics of such coherent nuclear motions, such as the oscillatory frequency and the dephasing time, differ between 1 and 2 and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt–Pt distance (2.97 A for 1 vs 2.85 A for 2) and molecular shape (1 in an “A” frame vs 2 in an “H” frame). Because the el...

Conformational analysis of an octahedral zinc(II) complex with six dimethylsulfoxide

An octahedral zinc(II) complex [Zn(DMSO)6](BPh4)2 (1) was synthesized (DMSO: dimethylsulfoxide), and its structure was determined by a single-crystal X-ray method. The ZnO6S6 unit has a pseudo-S6 symmetry, but the octahedral ZnO6 coordination geometry is slightly distorted, affording a pseudo-C2 symmetry. In order to clarify the reason for the distortion, conformational analysis was conducted for 130 conformers of [Zn(DMSO)6]2+ complex cation. The computationally obtained most stable structure was found to be partially deformed in the crystal in order to reduce the volume of the complex cation by crystal packing. The observed pseudo-C2 distortion was found to be caused by the interligand CH⋯O interactions.

Controlling Electron Transfer between the Two Cofactor Chains of Photosystem I by the Redox State of One of Their Components

Two functional electron transfer (ET) chains, related by a pseudo-C2 symmetry, are present in the reaction center of photosystem I (PSI). Due to slight differences in the environment around the cofactors of the two branches, there are differences in both the kinetics of ET and the proportion of ET that occurs on the two branches. The strongest evidence that this is indeed the case relied on the observation that the oxidation rates of the reduced phylloquinone (PhQ) cofactor differ by an order of magnitude. Site-directed mutagenesis of residues involved in the respective PhQ-binding sites resulted in a specific alteration of the rates of semiquinone oxidation. Here, we show that the PsaA-F689N mutation results in an ∼100-fold decrease in the observed rate of PhQA− oxidation. This is the largest change of PhQA− oxidation kinetics observed so far for a single-point mutation, resulting in a lifetime that exceeds that of the terminal electron donor, P700+. This situation allows a second photochemical charge separation event to be initiated before PhQA− has decayed, thereby mimicking in PSI a situation that occurs in type II reaction centers. The results indicate that the presence of PhQA− does not impact the overall quantum yield and leads to an almost complete redistribution of the fractional utilization of the two functional ET chains, in favor of the one that does not bear the charged species. The evolutionary implications of these results are also briefly discussed.

High-Resolution Electronic Structure of the Primary Electron Acceptor A0 of Photosystem I

The Type I photosynthetic reaction center, Photosystem I, is an exquisitely tuned protein complex comprised of multiple polypeptide subunits and protein-bound electron-transfer (ET) cofactors. Upon illumination, the special chlorophylls, P700, are photoexcited which results in the rapid formation of the charge-separated state, P700+A0-. In order to prevent charge recombination, the electron is transferred to the phylloquinone cofactor, A1, and subsequently to three four iron-four sulfur [4Fe4S] clusters, FA, FB, and FX, respectively. Through the ET reactions of PSI, the reducing equivalents that are required for the carbon fixation reactions are generated and stored as NADPH or ‘biohydrogen’. The PSI reaction center displays pseudo-C2 symmetry, such that the ET cofactors, A0 and A1, are duplicated in what is termed the A- and B-branch. Although ET is bidirectional along the A- and B-branch, it has been demonstrated that ET in the A-branch is highly preferred. This is thought to be due to the tuning of the redox-potential of the ET cofactors by smart matrix effects from the surrounding protein environment. This research explores the electronic structure of the primary electron acceptor A0 in the A-branch (A0A) through the use of two-dimensional (2D) hyperfine sublevel correlation (HYSCORE) spectroscopy in tandem with density functional theory calculations. The application of 2D HYSCORE spectroscopy allows for the identification of the 14N atoms and protons that are magnetically interacting with the paramagnetic center of A0A-. The 14N and 1H hyperfine parameters obtained here provide a direct measure of the electron spin density distribution of A0- in the A-branch of PSI. These are then be compared with simulations of the electronic structure of the A-branch of Photosystem I in order to better understand the mechanism of electron transfer from P700 to the primary electron acceptor, A0A.

Electric Field Asymmetry in the Photosynthetic Reaction Center?

The initial charge-separation steps of photosynthesis occur in the photosynthetic reaction center, which is a membrane-integral protein-pigment complex. The photosynthetic pigments comprise four bacteriochlorophylls, two bacteriopheophytins, two quinones, one carotenoid and one non-heme iron. Although these pigments are arranged in a pseudo C2-symmetry (L- and M-branch), electron transfer occurs almost exclusively along the L-branch. One possible explanation for this unidirectionality is based on the asymmetry of the protein electrostatic fields, which leads to a better stabilization of the charge-separated intermediates in the active L-branch. The vibrational Stark effect provides a way to measure electrostatic fields in proteins and we exploit this effect to compare electric fields between the L- and M-branch by using the carbonyl groups of the pigments involved in the electron transfer reaction.

Exploiting pseudo C2-symmetry for an efficient synthesis of the F-ring of the spongistatins.

A concise and efficient synthesis of the F-ring fragment of the potent antimitotic marine macrolide spongistatin 1 has been developed. The key sequence involves double cross-metathesis/Sharpless asymmetric dihydroxylation reactions to establish four stereocenters in a pseudo C2-symmetric array, followed by a selective protection reaction that breaks the pseudosymmetry, establishes a fifth stereocenter, and effectively differentiates the ester termini. Overall, the six contiguous stereocenters in the C(37)-C(45) F-ring fragment are established in just seven steps.

Influence of norbornanone substituents on both the Wagner–Meerwein skeletal rearrangements under sulfonation conditions and the diastereoselectivity of the corresponding N,N′-bis-fumaroyl sultams in uncatalyzed Diels–Alder cycloadditions to cyclopenta-1,3-diene

Abstract The Wagner–Meerwein domino rearrangement of norbornanone skeletons, under sulfonation conditions, is strongly influenced by the absence of a gem-dimethyl moiety at C(7). As a result, sulfonation at C(10) is less efficient due to a divergent pathway in the intermediate double bond formation and/or isomerization. Furthermore, the absence of such a gem-dimethyl moiety in the corresponding norbornane[10,2]sultam derivatives, sterically influences the orientation of the S O(1) and S O(2) substituents, hence on the π-facial steric shielding of the thermodynamically more stable anti-s-cis N-alkenoyl dienophiles. As a consequence, their diastereoselective [4+2] cycloadditions to cyclopenta-1,3-diene, under nonchelating conditions, are not as efficient due to a less pseudo axial S O(1) and the consequent loss of pseudo C2-symmetry.

Tuning of coordination geometry via cooperation of inter- and intramolecular hydrogen bonds in bis(benzoylacetonato)manganese(ii) adducts with pyridine derivatives

In order to study the effects of intramolecular hydrogen bonding on coordination geometry of a relatively rigid octahedral system, eight coordination compounds derived from bis(benzoylacetonato)manganese(II) and seven pyridine derivatives (three of them o-aminopyridines) were prepared. Four compounds in which intramolecular hydrogen bonds could not be formed are centrosymmetric with similar lengths of equivalent Mn–L bonds. When o-aminopyridines were employed, an intramolecular hydrogen bond was always formed. In the case of two compounds this led only to minor changes of molecular geometry, while in the other two the coordination complex was changed from a centrosymmetric all-trans configuration to a molecule with cis-configuration of pyridine ligands and of pseudo C2 symmetry. This change was found to affect not only the bond lengths of the coordination molecule, but also to allow for a much better crystal packing as it enabled a closer hydrogen bonding contact between adjacent molecules.

(Nitrato-κ2O,O′)bis(1,10-phenanthroline-κ2N,N′)copper(II) tricyanomethanide

The title compound, [Cu(NO3)(C12H8N2)2][C(CN)3], is formed of discrete [Cu(NO3)(phen)2]+ complex cations (phen is 1,10-phenanthroline) and C(CN)3 − counter-anions. The CuII atom has an asymmetric tetragonal–bipyramidal (4 + 1+1) stereochemistry with a pseudo-C 2 symmetry axis bis­ecting the nitrate ligand and passing through the CuII atom between the two phen ligands. The four N atoms of the phen ligands coordinate to the CuII atom with Cu—N distances in the range 1.974 (2)–2.126 (2) Å, while the two O atoms coordinate at substanti­ally different distances [2.154 (2) and 2.586 (2) Å]. The structure is stabilized by C—H⋯O hydrogen bonds and weak π–π inter­actions between nearly parallel benzene and pyridine rings of two adjacent phen mol­ecules, with centroid–centroid distances of 3.684 (2) and 3.6111 (2) Å, and between π-electrons of the tricyano­methanide anion and the pyridine or benzene rings [N⋯(ring centroid) distances = 3.553 (3)–3.875 (3) Å].

Exploring the Electron Transfer Pathways in Photosystem I by High-Time-Resolution Electron Paramagnetic Resonance: Observation of the B-Side Radical Pair P700+A1B– in Whole Cells of the Deuterated Green Alga Chlamydomonas reinhardtii at Cryogenic Temperatures

Crystallographic models of photosystem I (PS I) highlight a symmetrical arrangement of the electron transfer cofactors which are organized in two parallel branches (A, B) relative to a pseudo-C2 symmetry axis that is perpendicular to the membrane plane. Here, we explore the electron transfer pathways of PS I in whole cells of the deuterated green alga Chlamydomonas reinhardtii using high-time-resolution electron paramagnetic resonance (EPR) at cryogenic temperatures. Particular emphasis is given to quantum oscillations detectable in the tertiary radical pairs P700(+)A1A(-) and P700(+)A1B(-) of the electron transfer chain. Results are presented first for the deuterated site-directed mutant PsaA-M684H in which electron transfer beyond the primary electron acceptor A0A on the PsaA branch of electron transfer is impaired. Analysis of the quantum oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the geometry of the B-side radical pair. The orientation of the g tensor of P700(+) in an external reference system is adapted from a time-resolved multifrequency EPR study of deuterated and 15N-substituted cyanobacteria (Link, G.; Berthold, T.; Bechtold, M.; Weidner, J.-U.; Ohmes, E.; Tang, J.; Poluektov, O.; Utschig, L.; Schlesselman, S. L.; Thurnauer, M. C.; Kothe, G. J. Am. Chem. Soc. 2001, 123, 4211-4222). Thus, we obtain the three-dimensional structure of the B-side radical pair following photoexcitation of PS I in its native membrane. The new structure describes the position and orientation of the reduced B-side quinone A1B(-) on a nanosecond time scale after light-induced charge separation. Furthermore, we present results for deuterated wild-type cells of C. reinhardtii demonstrating that both radical pairs P700(+)A1A(-) and P700(+)A1B(-) participate in the electron transfer process according to a mole ratio of 0.71/0.29 in favor of P700(+)A1A(-). A detailed comparison reveals different orientations of A1A(-) and A1B(-) in their respective binding sites such that formation of a strong hydrogen bond from A1(-) to the protein backbone is possible only in the case of A1A(-). We suggest that this is relevant to the rates of forward electron transfer from A1A(-) or A1B(-) to the iron-sulfur center F(X), which differ by a factor of 10. Thus, the present study sheds new light on the orientation of the phylloquinone acceptors in their binding pockets in PS I and the effect this has on function.

Independent initiation of primary electron transfer in the two branches of the photosystem I reaction center

Photosystem I (PSI) is a large pigment-protein complex that unites a reaction center (RC) at the core with approximately 100 core antenna chlorophylls surrounding it. The RC is composed of two cofactor branches related by a pseudo-C2 symmetry axis. The ultimate electron donor, P(700) (a pair of chlorophylls), and the tertiary acceptor, F(X) (a Fe(4)S(4) cluster), are both located on this axis, while each of the two branches is made up of a pair of chlorophylls (ec2 and ec3) and a phylloquinone (PhQ). Based on the observed biphasic reduction of F(X), it has been suggested that both branches in PSI are competent for electron transfer (ET), but the nature and rate of the initial electron transfer steps have not been established. We report an ultrafast transient absorption study of Chlamydomonas reinhardtii mutants in which specific amino acids donating H-bonds to the 13(1)-keto oxygen of either ec3(A) (PsaA-Tyr696) or ec3(B) (PsaB-Tyr676) are converted to Phe, thus breaking the H-bond to a specific ec3 cofactor. We find that the rate of primary charge separation (CS) is lowered in both mutants, providing direct evidence that the primary ET event can be initiated independently in each branch. Furthermore, the data provide further support for the previously published model in which the initial CS event occurs within an ec2/ec3 pair, generating a primary ec2(+)ec3(-) radical pair, followed by rapid reduction by P(700) in the second ET step. A unique kinetic modeling approach allows estimation of the individual ET rates within the two cofactor branches.

Di-tert-butylN-[2,6-bis(methoxymethoxy)phenyl]iminodiacetate

The title mol­ecule, C20H31NO8, has pseudo-C2 symmetry about the C—N bond, with the bis­(tert-butoxy­carbon­yl)amino group twisted from the benzene ring plane by ca 60° and the bulky tert-butoxy­carbonyl (Boc) groups are orientated away from the substituted aniline group. As part of an anti­bacterial drug discovery programme furnishing analogues of platensimycin, we unexpectedly synthesized the bis-Boc-protected aniline.

Nanosized Pd37(CO)28{P(p-Tolyl)3}12 Containing Geometrically Unprecedented Central 23-Atom Interpenetrating Tri-icosahedral Palladium Kernel of Double Icosahedral Units: Its Postulated Metal-Core Evolution and Resulting Stereochemical Implications

Pd37(CO)28{P(p-Tolyl)3}12 (1) was obtained in approximately 50% yield by the short-time thermolysis of Pd10(CO)12{P(p-Tolyl)3}6 in THF solution followed by crystallization via layering with hexane under N2. The low-temperature (100 K) CCD X-ray diffraction study of 1 revealed an unusual non-spheroidal Pd37-atom polyhedron, which may be readily envisioned to originate via the initial formation of a heretofore non-isolated central Pd23 kernel composed of three interpenetrating trigonal-planar double icosahedra (DI) that are oriented along the three bonding edges of its interior Pd3 triangle. This central Pd23 kernel is augmented by face condensations with two additional phosphorus-free and 12 tri(p-C6H4Me)phosphine-ligated Pd atoms, which lower the pseudo-symmetry of the resulting 37-atom metal core from D(3h) to C2. The 12 P atoms and 28 bridging CO connectivities preserve the pseudo-C2 symmetry. The central Pd23 kernel in 1 provides the only crystallographic example of the 23-atom member of the double icosahedral family of "twinned" interpenetrating icosahedra (II), which includes the 19-atom two II (1 DI), the 23-atom three II (3 DI), the 26-atom four II (6 DI), and the 29-atom five II (9 DI). The n-atoms of these DI models coincide exactly with prominent atom-peak maxima of 19, 23, 26, and 29, respectively, in the mass spectrum of charged argon clusters formed in a low-temperature free-jet expansion. The only previous crystallographically proven 26- and 29-atom DI members are the central pseudo-T(d) tetrahedral Pd26 kernel (4 II, 6 DI) in the PMe3-ligated Pd29Ni3(CO)22(PMe3)13 (2) and the central pseudo-D(3h) trigonal-bipyramidal Pd29 kernel (5 II, 9 DI) in the PMe3-ligated Pd35(CO)23(PMe3)15 (3). Two highly important major stereochemical implications are noted: (1) The formation of geometrically identical idealized architectures for these three II palladium kernels with corresponding DI models constructed for the charged argon clusters provides compelling evidence that the nature of delocalized Pd-Pd bonding in these II (and presumably other nanosized) Pd clusters, in which each zerovalent Pd atom individually has a closed-subshell 4d (10) ground state, may likewise (as in argon clusters) be viewed primarily in terms of (considerably stronger) attractive dispersion interactions. (2) The existence of the 23-atom II Pd23 kernel in 1 provides an essential heretofore "missing" geometrical link as an intermediate in the same sequential growth pathway to give the 26- and 29-atom II Pd(n) kernels found in 2 and 3, respectively. Accommodation of the 12 bulky P(p-Tolyl)3 ligands around the entire 37-atom palladium core necessitates an extended metal surface that originates from the pseudo-2D trigonal-planar Pd23 kernel found in 1. The much smaller PMe3 ligands in 2 and 3 would sterically allow further sequential transformations of an initially formed 23-atom II intermediate palladium kernel into the 26-atom spheroidal II palladium kernel in 2 or further into the 29-atom semi-spheroidal II palladium kernel in 3, but with smaller total metal-atom nuclearities of 32 and 35, respectively.

Structure of Lumazine Protein, an Optical Transponder of Luminescent Bacteria

The intensely fluorescent lumazine protein is believed to be involved in the bioluminescence of certain marine bacteria. The sequence of the catalytically inactive protein resembles that of the enzyme riboflavin synthase. Its non-covalently bound fluorophore, 6,7-dimethyl-8-ribityllumazine, is the substrate of this enzyme and also the committed precursor of vitamin B2. An extensive crystallization screen was performed using numerous single-site mutants of the lumazine protein from Photobacterium leiognathi in complex with its fluorophore and with riboflavin, respectively. Only the L49N mutant in complex with riboflavin yielded suitable crystals, allowing X-ray structure determination to a resolution of 2.5 Å. The monomeric protein folds into two closely similar domains that are structurally related by pseudo-C2 symmetry, whereby the entire domain topology resembles that of riboflavin synthase. Riboflavin is bound to a shallow cavity in the N-terminal domain of lumazine protein, whereas the C-terminal domain lacks a ligand.

Highly Efficient Blue Photoexcitation of Europium in a Bimetallic Pt–Eu Complex

We report the preparation and characterization of dinuclear Pt-Ln complexes constructed from a square-planar Pt(II) core bearing an ethynyl-terpyridine residue connected to platinum by the ethynyl bond. Complexation of the neutral Eu(hfac)3 (hfac = hexafluoroacetylacetonate) fragment to free terpyridine (terpy) gives a stable bimetallic complex (log beta = 6.7). In the crystal structure, the flat Pt[triple bond]terpy core coordinates to Eu(III), which is nonacoordinated with the three nitrogen atoms of the terpy subunit and six oxygen atoms of the three hfac ligands. These atoms form a distorted monocapped square antiprism with a pseudo-C2 symmetry axis passing through the nitrogen atom of the central pyridine ring and the Eu atom. Spectroscopic measurements showed that irradiation with visible light of wavelength up to 460 nm in the 1MLCT state of the Pt subunit resulted in a quantitative energy transfer to the Eu center, which strongly luminesces in the red with an overall luminescence quantum yield of 38%. The energy-transfer process is quantitative and not sensitive to oxygen, and the complexation of Eu to the Pt metallosynthon allows the recovery of the energy lost due to triplet-oxygen quenching of the 3MLCT state observed in the uncomplexed Pt precursor.

Concise Synthesis of the (.+‐.)‐Nb‐Desmethyl‐meso‐chimonanthine.

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μ-Oxo-bis{isopropoxo[2,2′-(methylenedithio)bis(6-tert-butyl-4-methylphenolato)]titanium(IV)}

The title compound, [Ti2(C3H7O)2(C23H30O2S2)2O], contains an O,S,S,O-tetra­dentate bis­(phenolate) ligand. It is dimeric through a μ-O atom and exhibits pseudo-C2 symmetry. The two metal centres show a distorted octa­hedral coordination geometry.

A perfluorocyclopentene dithienylethene based molecular switch: 1,2-bis[5-(4-ethynylphenyl)-2-methylthiophen-3-yl]-3,3,4,4,5,5-hexafluorocyclopentene

The title compound, C31H18F6S2, is a mol­ecular switch based on dithienylperfluoro­cyclo­pentene. The mol­ecule possesses pseudo-C2 symmetry and has the open form of the switch. There are a number of intra­molecular C—H⋯F hydrogen bonds, but in the crystal structure there are no inter­actions between symmetry-related mol­ecules.